1. Field of the Invention
The present invention relates generally to coordination amongst execution sequences in a multiprocessor computer, and more particularly, to structures and techniques for facilitating implementations of concurrent data structures and/or programs.
2. Description of the Related Art
Management of dynamically allocated storage presents significant coordination challenges for multithreaded computations. One clear, but important, challenge is to avoid dereferencing pointers to storage that has been freed (typically by operation of another thread). Similarly, it is important to avoid modifying portions of a memory block that has been deallocated from a shared data structure (e.g., a node removed from a list by operation of another thread). These and other challenges are generally well recognized in the art.
A common coordination approach that addresses at least some of these challenges is to augment values in objects with version numbers or tags, and to access such values only through the use of Compare-And-Swap (CAS) instructions, such that if a CAS executes on an object after it has been deallocated, the value of the version number or tag will ensure that the CAS fails. See e.g., M. Michael & M. Scott, Nonblocking Algorithms and Preemption-Safe Locking on Multiprogrammed Shared Memory Multiprocessors, Journal of Parallel and Distributed Computing, 51(1):1-26, 1998. In such cases, the version number or tag is carried with the object through deallocation and reallocation, which is usually achieved through the use of explicit memory pools. Unfortunately, this approach has resulted in implementations that cannot free memory that is no longer required.
Valois proposed another approach, in which the memory allocator maintains reference counts for objects in order to determine when they can be freed. See J. Valois, Lock-free Linked Lists Using Compare-and-Swap, in Proceedings of the 14th Annual ACM Symposium on Principles of Distributed Computing, pages 214-22, 1995. Valois' approach allows the reference count of an object to be accessed even after the object has been released to the memory allocator. This behavior restricts what the memory allocator can do with released objects. For example, the released objects cannot be coalesced. Thus, the disadvantages of maintaining explicit memory pools are shared by Valois' approach. Furthermore, application designers sometimes need to switch between different memory allocation implementations for performance or other reasons. Valois' approach requires the memory allocator to support certain nonstandard functionality, and therefore precludes this possibility. Finally, the space overhead for per-object reference counts may be prohibitive. We have proposed another approach that does allow memory allocators to be interchanged, but depends on double compare-and-swap (DCAS), which is not widely supported. See e.g., commonly-owned, co-pending U.S. application Ser. No. 09/837,671, filed Apr. 18, 2001, entitled “Lock-Free Reference Counting,” and naming David L. Detlefs, Paul A. Martin, Mark S. Moir and Guy L. Steele Jr. as inventors.
Interestingly, the work that may come closest to meeting the goal of providing support for explicit non-blocking memory management that depends only on standard hardware and system support predates the work discussed above by almost a decade. Treiber proposed a technique called obligation passing. See R. Treiber, Systems Programming: Coping with Parallelism, Technical Report RJ5118, IBM Almaden Research Center, 1986. The instance of this technique for which Treiber presents specific details is in the implementation of a lock-free linked list supporting search, insert, and delete operations. This implementation allows freed nodes to be returned to the memory allocator through standard interfaces and without requiring special functionality of the memory allocator. However, it employs a “use counter” such that memory is reclaimed only by the “last” thread to access the linked list in any period. As a result, this implementation can be prevented from ever recovering any memory by a failed thread (which defeats one of the main purposes of using lock-free implementations). Another disadvantage of this implementation is that the obligation passing code is bundled together with the linked-list maintenance code (all of which is presented in assembler code). Because it is not clear what aspects of the linked-list code the obligation passing code depends on, it is difficult to apply this technique to other situations.